Structural performance

There are three elements to be considered in the structural design of vehicular trafficked pervious surfaces:

1. The surface course (porous asphalt, concrete slab, concrete blocks, reinforced gravel or grass). The function of the surface material is to provide good quality ride to be combined with appropriate skid resistance and resistance to crack propagation in the bituminous and concrete surfaces. Texture and durability under trafficking are key considerations.

2. The underlying construction layers which spread the applied loads (binder course, base, concrete slab and sub-base). These layers are required to provide a stable construction platform and spread the applied construction and traffic loading so that the underlying foundation layers are not over stressed.

3. The foundation soils (capping layer and subgrade). This is the platform upon which the structural layers are placed.

Each layer must be considered in turn and the key to successful structural design and performance is to recognise the difference between pervious and conventional pavements. Some factors to consider in the design and specification of each type of surface include:

Pervious pavements use materials with high permeability and void space. All the current structural pavement design methods commonly used in the UK are based on the use of conventional materials (which are dense and relatively impermeable). The stiffness of the materials to be used must, therefore, be assessed. This can be done based on equivalence.

Water is present within the construction and can soften and weaken materials and this must be allowed for, for example by measuring CBR values on saturated samples. The effects of freezing must be allowed for in the design of water storage layers. Specification of aggregates must allow for this.

The pavement designers should satisfy themselves that the materials to be used do not invalidate the assumptions made in the structural design methods.

The design methods assume full friction between layers. Any geotextiles or geomembranes must be carefully specified to minimise loss of friction between layers. Geotextiles and geogrids can be used to enhance the strength of pervious surfaces.

Porous asphalt loses adhesion (binder stripping) and becomes brittle as air passes through the voids. Its durability is therefore lower than conventional materials.

The single sized grading of the materials used means care must be taken to ensure that loss of finer particles between unbound layers does not occur. Geotextiles are important in this respect.

Block paving is one of the most common pervious surfaces constructed in the UK and may be designed using BS 7533. There is no current structural design method in the UK specifically for pervious pavements. They have, however, been in service in car parks in the USA for over 20 years and are used widely in Germany for applications such as bus and lorry parks, where heavy axle loads occur. Adverse structural effects have not been reported.

A common misconception with pervious pavements is that the presence of water in the unbound layers will reduce their strength and stiffness. Although this is true for materials such as Type 1 sub-base it does not apply to the single sized materials used in pervious pavements. Type 1 sub-base has a relatively high fines content and is therefore affected by changes in moisture content. The single sized nature of the sub-base required in a pervious pavement requires a low fines content. Although it will have a lower stiffness than Type 1, it will not to be significantly reduced further by the presence of water within it, provided there is sufficient friction between particles when saturated.

Pervious pavements require a single size grading to give open voids. The choice of materials for use in capping and sub-base layers below pervious pavements is therefore a compromise between stiffness, permeability and storage capacity.